This invention relates to controlling the backlighting of a video display. In particular, this invention relates to control of backlighting separately from the video display itself without introducing visual artifacts in the display.
In early video displays based on cathode-ray tube technology, the display generated its own light. However, in many types of solid-state video displays, the elements that display the video data do not generate their own light, and must be coupled with a separate light source. For example, liquid crystal displays operate by selectively lightening and darkening elements in an array, allowing light to shine through from behind the array. In such displays, the light source is generally a backlight, although for some displays, light may be provided from the sides, using reflectors, light pipes, etc., to spread the light out behind the liquid crystal array.
One type of light source commonly used in such displays includes fluorescent lamps. More recently, however, in order to reduce power consumption, and to save space, thereby allowing thinner displays, solid-state light sources have been introduced. For example, light-emitting diodes can be used, either in an array behind the liquid-crystal image-forming array, or as sidelights, with the light distributed behind the image-forming array using reflectors, light pipes, etc., as described above.
It is sometimes necessary to vary the brightness of a video display. This may be a function of the image being displayed, or it may be done to conserve power (e.g., in a portable device, the brightness may be reduced when operating on battery power, particularly during idle periods). One way of controlling the brightness is by pulse width modulation, in which a current pulse of duration t2 is sent during each interval of duration t1 to power the light source. Maximum brightness may be achieved when t2 is a certain fraction f of t1, where f≦1. By narrowing each pulse—i.e., shortening the pulse width, so that t2<ft1, the brightness can be reduced. The magnitude of each current pulse may remain constant.
The video array itself may have a certain refresh rate, which may be determined, for example, by the video standard being displayed, such as, e.g., NTSC, ATSC, VGA, SVGA, XVGA, etc. The video refresh rate may be totally independent of the pulse-width modulation pulse rate 1/t1. This complete lack of a fixed-phase relationship between the two signals may result in motion artifacts (e.g., a “waterfall” effect) in the video display when the refresh pulses do not coincide with the pulse-width modulated current pulses, for which there may be a number of solutions.
One solution is to synchronize the video refresh rate and the pulse-width modulation pulse rate of the backlight control current, so that the pulse-width modulation pulse rate of the backlight control current is an integer multiple of the refresh rate. This solution requires deriving both signals from a common clock source (e.g., a quartz crystal).
Another solution is to greatly increase the pulse-width modulation pulse rate of the backlight control current, so that when a video refresh pulse occurs, it will be very close to a backlight control current pulse so as to be nearly synchronous to the current pulse.
For example, a common video refresh rate is 60 Hz, while a common pulse-width modulation pulse rate for the backlight control current is 600 Hz. Either the 600 Hz backlight control current pulses may be synchronized with the 60 Hz refresh pulses, or the pulse-width modulation pulse rate of the backlight control current may be increased to between about 20 kHz and about 30 kHz. However, the video subsystem and the backlight subsystem are typically completely separate, so that synchronizing both rates using a common clock source is not practical or desirable, and very high pulse-width modulated backlight control current pulse rates also are not desirable.
Another solution might be to restart the pulse-width modulated backlight control current pulse train on each occurrence of a refresh pulse. However, because the pulse rate and the refresh rate may be completely independent, it may occur, by happenstance, depending on how the two cycles overlap, that a current pulse will have occurred just before a refresh pulse. Then, if the pulse train is restarted on occurrence of the refresh pulse, another current pulse will occur. The occurrence of two current pulses close together may cause a noticeable, if momentary, brightness increase or “flicker,” which also is undesirable.